1. Field of the Invention
The process and the apparatus which are the subject-matter of the present invention are concerned with the treatment of liquid steel in a ladle by means of gas injected from particular blowing elements which are suitably disposed in the bottom of the ladle to effect dehydrogenation, decarburization and renitridation of steels, in particular stainless steels, and decantation of killing inclusions.
2. Discussion of the Background
Treatment of liquid steels in a ladle by the injection of gas through the ladle bottom or by way of immersed lances has been used on an industrial scale for around twenty years for the purpose of homogenizing the temperature of the metal, desulphurizing it in contact with the slag and removing the killing inclusions.
The article by Grabner and Hoffgen "Einsatz und Verschleiss von Spulsteinen in der Sekundarmetallurgie" in Radex Rundschau issue 3, (1983), pages 179 to 209, reviews the conditions under which that procedure is usually carried into effect and its uses, the argon or nitrogen gas flow rates used as well as the design and the number of porous plugs used. The plugs are generally conical, being 1 or 2 in number, and they are generally disposed at two-thirds of the radius of the ladle, as determined from the center of the ladle. The total area of the porous plugs in contact with the liquid steel is between 25 and 190 cm.sup.2 depending on the size of the ladle and the flow rates used are between 3 and 10 liters per minute and per cm.sup.2 of surface area of porous plug, in contact with the liquid metal.
Electric Furnace Steel Making, volume II, Theory and Fundamentals, by D. C. Hilty, R. W. Farley and D. J. Garde, edited 1967 by E. Sims, also sets out the thermodynamic elements required to understand the mechanism of decarburization under partial CO pressure, (pages 124, 125 and 171 to 175) for steels with a high proportion of chromium.
The Revue de Metallurgie, January (1986), pages 25 to 41, by C. Gatelier and H. Gaye sets out thermodynamic considerations relating more particularly to the exchanges between the liquid steel and the hydrogen and nitrogen gases, the dimensioning of the bubbles of gas emitted as well as the method of calculating their rate of rise.
However, in spite of the efforts involving increasing the gas flow rates conventionally used with the known porous plugs and increasing the duration of the treatment to the limits permitted by the temperature losses in the ladle, it is not possible to provide either for complete decantation of the inclusions or sufficient dehydrogenation or renitridation of nitrogen-bearing stainless steels to the desired value or decarburization under a low partial CO pressure in order to achieve very low proportions of carbon which are below 0.025% by weight for stainless or carbon steels. The volumes of the argon-oxygen mixture are such that the conventional porous plugs do not provide an adequate flow rate and the rise in temperature at the location of the plugs resulting from combustion is also excessive.
Thus, for the purposes of dehydrogenating steels, the present procedure in its general principle involves subjecting the metal to a vacuum, generally about 1 Torr, with the layers of steel being constantly renewed so that the partial pressure of hydrogen dissolved in the metal is always higher than that of the hydrogen in the level of vacuum in question so that the hydrogen can diffuse.
Although renitridation of nitrogen-bearing stainless steels (0.1 to 0.4% by weight nitrogen) is already effective in the AOD converter (Argon Oxygen Decarburizing process), by virtue of replacing argon with nitrogen as the gas for dilution of the CO formed, it is not sufficient and must be supplemented by final additions, generally of nitrided ferrochromium, which are highly expensive.
The production of steels with a very low carbon content of less than 0.025 wt% or steels with a high chromium content in which combustion of the carbon must be effected under a partial CO pressure of less than unity, the pressure being dependent on the carbon content and temperature, in AOD converters or in vacuum degassing installations in order to limit the degree of oxidation of the metal. In AOD converters, the partial CO pressure is achieved by dilution. In vacuum degassing installations, oxygen is injected by means of a lance at the desired pressure to produce the desired CO pressure. These apparatuses are frequently referred to as RH-OB or VOD in the specialist literature.
At the present time, bearing steels, after a first treatment to decant the inclusions in the ladle, are subjected to a complementary operation generally with an elevator under vacuum (R.H.) in which the rate of circulation of the metal is very high. This circulation takes place under turbulent flow conditions, which increases the probability of elementary inclusions of alumina, the size of which is close to a micron, agglomerating and being of a sufficient size to decant in the liquid steel by virtue of a difference in density or clinging to the refractory wall.
French Pat. No. 2,223,467 discloses a process for circulating the whole of a cast iron bath, by pneumatic means, in a ladle, and not in an apparatus under vacuum. This process is concerned with introducing desulphurizing agents such as calcium carbide or graphitization innoculating agents, into the very heart of the mass of metal. As these agents are of a density which is two to three times less than that of the metal, they impose on the central downwardly directed flow of metal, a speed which is greater than that of the upward movement of the agents, thereby imposing a porous ring configuration. The width of the configuration can attain a quarter of the inside diameter of the ladle, or three quarters of the surface area of the bottom of the ladle.
The object of the present invention is entirely different from that which French Pat. No. 2,223,467 seeks to attain, since the present invention seeks to multiply the metal-gas exchange surfaces while retaining the individuality of each of the small bubbles emitted and their low rate of rise, while also seeking to concentrate the slag at the center of the free upward surface of the liquid metal so that the slag cannot be entrained right within the liquid steel. This requires mixing surface areas which are very much smaller than the above-indicated surfaces, very specific gas flow rates, precise positioning of the stirring elements and a suitable level of porosity.
The problems of the above-discussed industrial installations are that they require very high levels of capital investment, giving rise to high levels of heat losses, often requiring reheating of the metal either by alumino or silico-thermy (RH-OB and AOD), or by means of electric arcs in the ladle. The treatment costs are therefore high.